As critical data is increasingly stored in electronic form, it is imperative that it be stored reliably in a tamper-proof manner. Furthermore, a growing subset of electronic data (e.g., electronic mail, instant messages, drug development logs, medical records) is subject to regulations governing its long-term retention and availability. Recent high-profile accountability issues at large public companies have further caused regulatory bodies such as the Securities and Exchange Commission (SEC) to tighten their regulations. For instance, SEC's new Rule 17a-4, which went into effect in May 2003, specifies storage requirements for email, attachments, memos and instant messaging as well as routine phone conversations. The key requirement in many such regulations is that data must be stored reliably in non-erasable, non-rewritable storage such that once the data is written, it cannot be altered or overwritten. Such non-rewritable storage is commonly referred to as WORM (Write-Once Read-Many) storage as opposed to WMRM (Write-Many Read-Many) storage, which can be written many times.
Traditionally, WORM storage is implemented by using optical means to cause an irreversible change in a storage medium (e.g., WORM optical disc, CD-R, DVD-R). Because of market forces and physical and/or technological constraints, WORM optical storage has not been improving much and is unlikely to improve much in performance and/or storage density. Organizations thus must store their rapidly growing volumes of critical records on an increasingly large number of WORM optical discs. Managing such a large number of discs is a massive, time-consuming, error-prone and expensive process. More importantly, records pertinent to a data discovery request cannot be easily located within such a system and delivered in a timely fashion.
A recently introduced alternative to WORM optical storage is the WORM tape. The WORM tape is slow and does not allow direct random access to the data. Instead, data has to be appended and read sequentially. Although write-once (reference) data is sometimes described as read-rarely or read-never, there are many environments where read performance does matter. Moreover, when discovery requests for data arrive, the data needs to be readily accessible, in random and sequential modes depending on the application.
Another alternative to WORM optical storage is content-addressable storage (CAS). CAS achieves the effect of write-once storage by storing data at a location that is uniquely determined by the data being written. Such a system, however, requires a new storage interface and extensive changes to applications and system software to carefully track and manage the locations at which data is written.